OBJECTIVE To define the developmental potential of marmoset ES cells in vivo, and to test the potential of marmoset ES cell nuclei to support development when transferred to enucleated oocytes. RESULTS During 1998, we tested a series constructs for use as lineages markers to identify marmoset ES cells in chimeras. We identified ovarian hyperstimulations protocols that allow the recovery of multiple oocytes per female marmoset, and developed IVF and culture conditions that allow in vitro development of marmoset embryos to the blastocyst stage.. FUTURE DIRECTIONS We will use expression constructs to analyze the tissue distribution of ES cell derivatives in chimeric marmoset fetuses, and thus define the developmental potential of marmoset ES cells. We will begin nuclear transfer experiments to produce genetically identical marmosets from ES cells. KEY WORDS Callithrix jacchus, chimera, embryonic stem cells, blastocyst FUNDING NIH 1R24RR11571 PUBLICATIONS Thomson, J.A. and Marshall, V.S. 1998. Primate Embryonic Stem Cells. Curr. Top. in Dev. Biol. 38:133-160.